Embodiments described herein relate generally to electric double layer capacitors having an electrolyte formulation that includes a stabilizing additive such that the electric double layer capacitor retains its cell capacitance for longer periods of time, and ESR gain and gas generation are reduced during operation of the cell.
Electric double layer capacitors (EDLCs) which are also referred to as ultracapacitors or supercapacitors, generally enable fast delivery of electrical energy in a short period of time. Thus, EDLCs are well suited for high power applications such as, for example, PC cards, CF cards, mini PCI, express cards, USB modems, toll tags, energy backup in portable or energy constrained devices (e.g., UPS), voltage regulation for CPU's, supplemental power in hybrid electric vehicles, portable audio and other devices with high surge loads, actuators for door lock, LED flash cameras, solid state memory devices, and other devices requiring high power and high energy. In particular, EDLCs can be used as an integral energy storing component of the drive trains of hybrid electric vehicles. Known hybrid electric vehicles use EDLCs as the primary power source during acceleration and for storage of energy during regenerative breaking.
EDLCs include an anode and a cathode (e.g., carbon based anode and carbon based cathode) that are separated by a separator. The EDLC is infiltrated with an electrolyte such as, for example, an aqueous electrolyte (e.g., sulfuric acid solutions, potassium hydroxide solutions, etc.) or a non-aqueous electrolyte. Known electrolytes typically include an aqueous or non-aqueous solvent which contains one or more dissolved ionic species. Known electrolytes such as, for example, aqueous electrolytes can be unstable at normal operating conditions of the EDLC. An additive, for example, a stabilizer can be added to such known unstable electrolytes to stabilize the electrolyte for operation at normal EDLC working conditions.
Known EDLCs also use electrolytes which are stable at normal operating conditions of the EDLC. Such stable electrolytes can include, for example, non-aqueous electrolytes that include a non-aqueous solvent such as, for example, acetonitrile or propylene carbonate, and a highly stable ionic species such as, for example a quaternary ammonium tetrafluoroborate salt. Known stable electrolytes can, however, become unstable under adverse operational conditions, for example, high voltage or high temperature conditions. As the demand for EDLC grows, there is a continuing need for new solvents and electrolyte systems that exhibit better stability and operational characteristics.
Thus, it is an enduring goal of EDLC development to develop new electrolyte formulations and EDLCs that have higher stability and longer life.